Inhibitory nucleic acids downregulate gene expression through a variety of mechanisms. Antisense oligonucleotides, for instance, are typically single strands of DNA or RNA that are at least partially complementary to a chosen target sequence and function by preventing protein translation of specific messenger RNA strands. Double-stranded RNA (dsRNA) is another type of inhibitory nucleic acid that can induce sequence-specific posttranscriptional gene silencing in many organisms by a process known as RNA interference (RNAi). These functional RNAs include at least two types of small RNA molecules (1) siRNA molecules that induce mRNA degradation, and (2) miRNAs (microRNAs) that induce translational inhibition. Other small RNAs work at the transcriptional level by affecting DNA and histone methylation.